1. Field of the Invention
The present invention relates to a transmitting apparatus, a transmitting method, a receiving apparatus, a receiving method, a transmitting and receiving apparatus, and a transmitting and receiving method suitable for broadcasting hierarchical data that is distributed on a network.
2. Description of the Related Art
Many data delivering methods have been proposed. For example, http (Hyper Text Transfer Protocol) is used to publish pages on the Internet. TCP/IP (Transmission Control Protocol/Internet Protocol) enables computers connected to the Internet to exchange data. In the TCP/IP, a receiving side that receives data calls a transmitting side. Whenever data is transmitted or received corresponding to the TCP/IP, a connection is established between the receiving side and the transmitting side. Thus, with such protocols, data can be delivered with high reliability. On the other hand, the transmitting side and the network are adversely loaded and thereby data cannot be effectively delivered.
In other words, when the number of terminal units that receive data is becoming large and they access a server that delivers data thereto at a time, the server and the network are adversely loaded. Thus, even if a terminal unit requests the server for data, the terminal unit cannot receive the requested data from the server in a short time.
To solve such a problem, a method using a satellite line, a CATV (Cable Television) line, a ground wave digital broadcast, and so forth that allows data to be broadcast has been proposed. With this method, even if the number of terminal units increases, the server and the network are prevented from being adversely overloaded.
In recent years, as digital communication networks such as the Internet have become common, a huge amount of data has been stored on the networks. Thus, it is desired to effectively use such data. To do that, a directory service for hierarchically managing data distributed on a network and providing the data to the user is becoming popular. Using the directory service, the user can quickly find desired information from data distributed on the network and access the desired information.
The directory service has been set forth as X.500 series in OSI (Open System Interconnection) that is an international standard. In the X.500, the directory is defined as a set of open systems. Individual open systems cooperatively have logical databases of information with respect to a set of objects of the real world.
With major directory services defined in the X.500, the user can search and browse information stored in the directory. The directory services also provide the user with a list service (such as a telephone directory) and a user authenticating service. In the directory service, each object is assigned a unique name so that the user can easily memorize, infer, and recognize each object.
The directory services defined in the X.500 are very comprehensive. The program size of each directory service is very large. Thus, it is very difficult to accomplish a directory service on the Internet that uses the TCP/IP as a protocol. To solve such a problem, LDAP (Lightweight Directory Access Protocol) has been proposed as a compact type directory service for the TCP/IP.
In recent years, a directory service using a broadcast data transmitting means such as a satellite line, a CATV line, a ground digital broadcast, or the like has been proposed. In this case, information is uni-directionally delivered with the directory service. Thus, the user cannot request the directory service for desired data. Consequently, in such a directory service, the same information is repeatedly transmitted. The user side stores received information to an IRD (Integrated Receiver Decoder) or an STB (Set Top Box) that is a digital broadcast receiver connected to a television receiver.
Next, information synchronization between the directory server side and the user side (namely, synchronization management) will be considered. The directory server side detects a substantial update of the directory corresponding to a dynamic change of the hierarchical structure of the directory. The directory server side transmits directory information corresponding to the change of the hierarchical structure of the directory to the user. At this point, only the difference of the changed data rather than all directory information is extracted and transmitted.
The directory server side transmits difference update data to the user whenever the directory server side detects an update of the directory information or at intervals of a predetermined time period. When the user side always receives the difference update data corresponding to an update of the directory and updates the stored directory information with the received difference update data, the directory information is synchronized between the directory server and the user.
Now, it is assumed that the user side is in a pause state (for example, the power of the receiver is turned off) and cannot receive difference update data from the directory server for a predetermined time period. For example, as shown in FIG. 1, the directory server successively transmits difference update data Msg1, Msg2, and Msg3 to the user. The difference update data Msg2 is the difference of the directory information updated by the difference update data Msg1. The difference update data Msg3 is the difference of the directory information updated by the difference update data Msg2.
As represented with a middle portion of FIG. 1, if the receiving side is in the pause state, the receiving side cannot receive the difference update data Msg2. In this case, the receiving side updates the difference update message Msg1 with the difference update data Msg3. Thus, on the receiving side, the directory data cannot be correctly updated. Consequently, the directory data of the directory server does not match the directory data of the receiving side.